Modeling Deep Rooted Thrust Mechanism of Crustal Thickening in Eastern Tibet

• Published in: Geophysical research letters Vol. 50; no. 15
• Main Authors: Pitard, P., Replumaz, A., Thieulot, C., Doin, M.‐P.
• Format: Journal Article
• Language: English
• Published: Washington John Wiley & Sons, Inc 16.08.2023; American Geophysical Union; Wiley
• Subjects: Barriers; Cratons; crustal thickening Eastern Tibet; Crustal thickness; Deformation; Discontinuity; ductile lower crust; Earth Sciences; Geophysics; Inversions; Kinematics; localisation of deformation; Mathematical models; Modelling; Numerical models; Orogeny; Overpressure; Plateaus; Sciences of the Universe; Shear zone; Thickening; upper crust thrust embedded; Viscosity; Tibet; Mechanical modelling; Tibet Plateau; crustal fault
• ISSN: 0094-8276; 1944-8007
• DOI: 10.1029/2023GL104134

To test Eastern Tibet crustal thickening modes, we compare 2‐D numerical models of two emblematic end‐member models, with either an obstacle in the low viscosity lower crust or a thrust embedded in the high viscosity one. We show that the obstacle halts the viscous lower crustal flow potentially initiated by the weight of the high Central Tibet, generating a smooth exhumation gradient at the edge of the plateau, not observed in Eastern Tibet. On the contrary, including a low viscosity discontinuity in the upper crust, mimicking a shallow steep listric fault as inferred in the region, reproduces a sharper exhumation profile, as constrained from thermo‐kinematic inversions of thermochronological data, and the lack of foreland basin, as observed in the field. Moreover, such fault drives deformation throughout the entire crust, suggesting a deep crustal ductile shear zone limiting the more ductile deformation in the lower crust although no discontinuity is imposed. Plain Language Summary The role of thrusting in crustal thickening during the formation of Tibet, the world's largest and highest orogenic plateau, constitutes one of the main controversies in earth sciences. In Eastern Tibet in particular, two end‐members based on two contrasting controversial hypotheses can be tested: the thickening is dominated either by the flow of the lower Tibetan crust halted by the hard Sichuan craton, or by thrusting of the Tibetan upper crust. Here, we present 2‐D crustal numerical models of a shallow steep listric thrust (as inferred in the region) embedded in the high viscosity upper crust, and we show that such model reproduces the exhumation profile constrained from thermochronological data and the lack of foreland basin observed in the field. Interestingly, we also show that such upper crustal thrust drives upward the more ductile lower crust albeit no discontinuity is imposed. On the contrary, by using a model driven by an overpressure in the lower crust, we show that the obstacle halts the viscous lower crustal flow and generates a smooth exhumation gradient at the edge of the plateau, not observed in Eastern Tibet. Key Points 2‐D numerical models of thrusts embedded in the high viscosity upper crust, to test thermo‐kinematic models based on thermochronology data accommodation in the lower crust by ductile flow of the deformation induced by the high angle thrust in the upper crust predicting exhumation rates and subsidence patterns that are compatible with the measured ones in Eastern Tibet